When you're writing an adventure, don't think of yourself as writing a book for a casual reader (despite the fact that many of your readers will, in fact, be casual readers). Instead, think of yourself as writing for your fandom: a group of people who will dissect everything you provide, question it, review it, and build on it all on their own. This means you should keep a few things in mind:

* Make motives crystal clear. When an NPC does something, spend the words to make sure you're clarifying why. If you expect the heroes to do something, explain what it is and why. Fans particularly dislike characters that act erratically with no apparent motive, or in contradiction to previously established motives. The answer is to be clear with these--and if you're unclear yourself, your writing will clarify this.

* Expect minor character to take a major role. One thing that continually surprises me when I hear people talk about playing adventures I've written is how much the GM and players have emphasized NPCs that I felt like had a really, really minor role. A casual hobgoblin boss or robbery victim's son might be the NPC the players and GM all best remember. This means you should have some care with even your minor characters: make sure they're at least described with an interesting hook or tow, and maybe explain how they might have a further role in the adventure if the GM decides it.

* Allow places to go off the map. GMs are endlessly innovative. They can make exceptionally good use out of places in your adventure that aren't very well described, or side areas with only minimal hints about what's there. You should definitely include these in your adventure to allow for this. I do so by having dungeon passages that go off the side of the map, or by tagging areas in an overland map that are only barely hinted at in the text. These give places GMs can expand. Of course, you should identify what you're doing clearly in the text, so it doesn't look like you just forgot to explain something. Indicate you intentionally left this open with language like "the GM can explore this further, especially if the gnolls in area 13 survive and flee this way" or the time-honored phrase that the sprawling network of passages at the base of this stairway (or whatever) are "beyond the scope of this adventure."

This type of writing gives your readers more to play with, and that's fun for them!